1. Field of the Invention
This invention relates to electronic and electrochemical apparatus and methods for measuring the concentration of fluoride ions in aqueous liquid compositions. The general principles of the apparatus are described in U.S. Pat. No. 3,431,182 of Mar. 4, 1969 to Frant, the entire disclosure of which, except to the extent that it may be contrary to any explicit statement herein, is hereby incorporated herein by reference. This invention is more particularly related to modifications that prolong the service life of the electrodes, particularly when the latter are used in contact with hot acidic aqueous compositions containing surfactants and/or oxidizing agents; such compositions are customarily employed in cleaning aluminum beverage containers in a high speed processing line and in forming protective coatings on aluminum surfaces generally.
2. Discussion of Related Art
One of the necessary parts of apparatus according to this invention and the prior art is a substantially liquid impermeable membrane that is constituted of ion-sensitive crystalline fluoride(s). In a complete free fluoride ion sensitive electrode, this membrane is physically arranged to have one surface of the membrane contacted by an ionically conductive liquid in which the concentration of free fluoride ions is to be measured with the aid of the free fluoride ion sensitive electrode. The other side of the membrane is in indirect electrical contact, via physical contact with a first liquid ionic solution of known composition, with a first "reference electrode", i.e., an electrode, such as a silver-silver chloride electrode, that automatically maintains itself at a fixed potential as long as the composition of the ionic solution with which it is in contact does not change and the current density flowing through it is small. The first ionic solution of known composition, the first reference electrode, and the membrane described above are all assembled within and/or on the surface of a container, usually a tubular container, in such a manner that (i) the membrane has one side in contact with the first ionic solution of known composition and its other side on the outside of the container, (ii) the first liquid ionic solution of known composition is sealed within the container, (iii) the first reference electrode is in physical contact with the first liquid ionic solution of known composition and is not in physical contact with the membrane, but is electrically connected thereto via the first liquid ionic solution of known composition; and (iv) the metallic part of the first reference electrode is connected through a metallic electrical conductor lead to the space outside a single sealed space within the container that contains a metallic part of the first reference electrode, the first liquid ionic solution of known composition, and the interior side of the solid fluoride(s) membrane. The Frant reference teaches that the container may be made of any liquid-impervious, substantially rigid, electrically insulating material that is substantially chemically inert to salt solutions containing fluoride ions with which the container might be placed in contact, and in practice most if not all commercially available free fluoride ion sensitive electrodes have plastic containers, usually of poly{phenylene sulfide}, with or without inorganic fillers.
When a free fluoride ion sensitive electrode is in use, the electrical lead from the container for the first reference electrode is connected via a metallic conductor to a low current voltage measuring device and through the latter to a second reference electrode, which may or may not be at the same inherent, self-maintained potential as the first reference electrode. The second reference electrode is in contact with a second liquid ionic solution of known composition, which may or may not be the same as the composition of the first liquid ionic solution, and the second liquid ionic solution is not in physical contact with, but is in indirect electrical contact with, the aqueous composition containing the concentration of free fluoride ions to be measured, the indirect contact being established through one of the devices known in the art, such as a glass fiber, gel of ionic solution, porous TEFLON.TM. polytetrafluoroethylene plug as described, for example in U.S. Pat. No. 4,128,468 of Dec. 5, 1978 to Bukamier, "flowing junction", or the like, which permits electrical conductance via ion charge transfer while preventing any substantial physical mixing between the second liquid ionic solution of known composition and the liquid in which the concentration of free fluoride ions is to be measured.
Although fluoride sensitive electrodes of this type have been in use for decades, it has been widely recognized in the art that one of their major disadvantages is an instability of performance. Such instability is usually first manifested as a change in the voltage reading that corresponds to a particular value of free fluoride ions, necessitating frequent recalibration of the particular fluoride sensitive electrode used. Eventually, the electrode may become non-responsive to changes in fluoride concentration within the range in which measurement is needed for the process control of the particular solution being monitored with the electrode, so that the electrode must be replaced. This is a substantial source of expense and also of inconvenience and hazard, inasmuch as many of the solutions in which fluoride concentrations are measured contain hazardous ingredients such as hot hydrofluoric acid and the free fluoride ion sensitive electrodes are often mounted in the wall of a container vessel for the process solution being monitored, well below the highest liquid level in the vessel; considerable care must be exercised when replacing any such component in contact with such a solution in order to avoid injury.
Most prior art attempts to improve the life of free fluoride ion sensitive electrodes have been directed to improving the quality of the seal between the solid fluoride(s) membrane and the container wall in which the membrane is fixed, in order to prevent or at least minimize opportunities for fluid and/or electrical leaks through this desirably completely sealed interface. For example, U.S. Pat. No. 4,049,526 of Sep. 20, 1977 to Maurer teaches that a slow setting type of cement is practically required for sealing the solid fluoride(s) membrane to the insulating container, but that such a slow-setting cement has poor long term resistance to many solutions in which measurement of the free fluoride ion concentration with the free fluoride ion sensitive electrode is desired, so that the slow setting cement, even after it is completely set, should be protected from these solutions with a fast setting cyanoacrylate cement shield.
More recently than the issue date of the Maurer patent, extensive use has been made of a VITON.TM. or like fluorocarbon elastomer O-ring or gasket to protect the integrity of the slow setting cement interface between the solid fluoride(s) membrane and the insulating container wall. The solid fluoride(s) membrane normally is cemented into a recess provided in the insulating container wall, so that the bottom surface of the assembly is a flat circular disk that has the solid fluoride(s) membrane as its center, surrounded by an annular ring of cement which itself is surrounded by an annular ring of the material of which the major insulating part(s) of the container wall are constructed. The exterior of the insulating container wall immediately at and away from this flat circular disk is threaded for a sufficient distance to hold in place a retaining collar, with corresponding female threading on its interior cylindrical surface and a circular hole through its flat surface. The sizes of the circular holes in the retaining collar and in the elastomeric O-ring or gasket are at least slightly smaller than the size of the exposed exterior surface of the solid fluoride(s) membrane, and the exterior diameter of the O-ring or gasket is small enough to fit within the retaining collar and large enough to be retained thereby when the latter is screwed into place over the threaded end of the insulating container exterior, so that the O-ring or gasket is compressed and forms a seal that at least initially is liquid tight. However, neither this nor any other expedient known to prior art has proved to be fully satisfactory for assuring long term reliability for the free fluoride ion sensitive electrodes now in general use.